U.S. patent number 8,220,693 [Application Number 12/614,489] was granted by the patent office on 2012-07-17 for modified surfaces using friction stir processing.
This patent grant is currently assigned to GM Global Technology Operations LLC. Invention is credited to Yen-Lung Chen, Paul E. Krajewski.
United States Patent |
8,220,693 |
Krajewski , et al. |
July 17, 2012 |
Modified surfaces using friction stir processing
Abstract
A procedure for incorporating particles to a predetermined depth
in a body to develop a composite surface layer using friction stir
processing is described. The process comprises; preparing the
surface of the body; applying particulate matter to the prepared
surface; selecting a friction stir tool with a pin length
substantially equal to the desired composite layer thickness; using
a friction stir process to frictionally heat, stir and intermix the
particulate matter with the matrix of the article; and traversing
the friction stir tool across the body.
Inventors: |
Krajewski; Paul E. (Troy,
MI), Chen; Yen-Lung (Troy, MI) |
Assignee: |
GM Global Technology Operations
LLC (Detroit, MI)
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Family
ID: |
43974390 |
Appl.
No.: |
12/614,489 |
Filed: |
November 9, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110111246 A1 |
May 12, 2011 |
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Current U.S.
Class: |
228/112.1;
228/199; 228/159 |
Current CPC
Class: |
B23K
20/122 (20130101); B23K 20/1215 (20130101); C23C
24/06 (20130101); C22C 1/1084 (20130101); C23C
26/00 (20130101); B22F 7/04 (20130101); Y10T
428/12993 (20150115); Y10T 428/12007 (20150115) |
Current International
Class: |
B23K
20/12 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2009/142070 |
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Apr 2009 |
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WO |
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Primary Examiner: Stoner; Kiley
Attorney, Agent or Firm: Reising Ethington P.C.
Claims
The invention claimed is:
1. A method for selectively modifying at least a portion of a metal
sheet metal forming die body by creating a composite, lubricious
layer of predetermined depth in a surface of the body intended for
contact with the sheet metal, the surface for creating such layer
protruding beyond the intended die body surface; the method
comprising: forming indentations protruding in the surface of the
body in which the composite layer is to be formed, the indentations
being sized and spaced to accept and store a predetermined quantity
of particles of a desired material for incorporation of the
material in the surface of the die body; the surface protruding
beyond the intended die body surface to provide excess body
material for shaping and suitably finishing the modified surface;
placing a predetermined quantity of at least one particulate
material in the indentations in the surface of the body; using a
friction stir tool with a pin, the pin having a length and a
diameter, to frictionally heat and mix the particulate matter in
the indentations with adjacent metal of the body and traversing the
friction stir tool across the body to mix the particulate material
with the material of the body to a depth substantially equal to the
length of the pin, such mixture constituting a precursor of the
intended composite layer; and then performing one or more of
machining, grinding and polishing on at least the modified portion
of the precursor surface to form the die body and render, on the
modified surface portion of the die body, a suitably shaped and
lubricious surface for sheet metal contact.
2. The method of claim 1 where indentations are formed in the
surface of the body by a process selected from the group consisting
of milling, sawing, knurling, shot blasting, grinding, turning or
drilling.
3. The method of claim 1 where the particulate material is a hard
material selected from the group consisting of aluminum oxide,
silicon carbide and cubic boron nitride.
4. The method of claim 1 where the particulate material is a
lubricious material selected from the group consisting of boron
nitride, molybdenum disulphide and graphite.
5. The method of claim 1 where the predetermined quantity of
particulates is selected to achieve a desired concentration of
particulates in the composite layer.
6. The method of claim 1 where the particles have a size and the
particle size is less than the size of the indentations.
7. A method for selectively modifying at least a portion of a
surface layer of a die body as recited in claim 1 wherein the
composite layer enhances the wear resistance of the surface
layer.
8. A method for selectively modifying at least a portion of a
surface layer of a die body as recited in claim 1 wherein the
composite layer enhances the lubrication properties of the surface
layer.
9. A method for selectively modifying at least a portion of a
surface layer of a die body as recited in claim 1 in which the
metal body is one of a cast iron or a tool steel.
10. A method for selectively modifying at least a portion of a
surface layer of the die body as recited in claim 1, in which the
pin diameter is greater than the spacing between indentations and
the tool traces a path to process the entire surface of the body
containing the particulate, material-containing indentations.
Description
TECHNICAL FIELD
This application relates to processes for selective modification of
surfaces through friction stir methods. More specifically, this
disclosure pertains to the incorporation of surface modifying
particles into workpiece surfaces using a friction stir processing
tool.
BACKGROUND OF THE INVENTION
The forming of automotive body panels such as doors or decklids is
accomplished through the interaction of an initially planar blank
with complementary, contoured forming tools, or dies, under the
urging of a load applied by a forming press. The load distribution
on the dies is highly non-uniform, with some regions of the dies
experiencing high loads while others are only lightly loaded.
Because there will be at least some relative motion between the
sheet metal blank and the dies, the magnitude of the local applied
load significantly influences the local interaction between the
blank and the dies.
Under high load, the interaction between sheet and die can lead to
die wear which can modify the die surface finish and die geometry
as well as to transfer of material from the blank to the die,
usually described as galling or `die pick-up`. Galling and `die
pick-up` are different, but related mechanisms. The transfer of
aluminum from a workpiece to a dies can lead to accumulation of
material, resembling a particle on the surface of the die. This is
`die pick-up`. Once a particle is present on the die surface, it
can scratch or gall subsequent panels. Galling can lead to
additional material transfer or pick-up.
These undesired outcomes have been managed by increasing the die
hardness, frequently though surface coatings, or by application of
lubricants or combinations of these approaches. However, such
coatings and lubricants are expensive; frequently create
housekeeping issues related to their application; and convey only
temporary benefit thereby necessitating frequent re-application to
sustain their effectiveness.
Thus there is a need for processes capable of imparting more
durable lubricating surface to dies to better manage the sheet
forming process.
SUMMARY OF THE INVENTION
This invention provides a practice for modifying the surface of a
workpiece to incorporate particles of a material that desirably
alter the properties of the surface. For example, the workpiece may
be a die or other forming tool to which enhanced surface
lubrication properties or wear resistance properties may be
imparted. In other examples, the workpiece may be used in sliding
contact or force imparting contact with another article. The
workpiece will typically be formed of a metal alloy.
In accordance with embodiments of the invention, a surface of the
workpiece is modified to a predetermined depth to receive and
temporarily hold particles of a selected surface modifying
material. Examples of surface modifying materials include particles
of aluminum oxide, boron nitride, cubic boron nitride, graphite,
molybdenum disulfide, silicon carbide, and the like. The surface of
the workpiece is prepared with particle receptacle cavities such as
dimples, pits, slots, and the like by surface machining such as
knurling, shot blasting, grinding, drilling, milling, sawing, or
the like. When the workpiece surface has been roughened to a depth
to hold the particles and achieve a desired surface layer
thickness, the particles are placed in the surface receptacles in a
suitable quantity. The particles are then worked into the surface
by a friction stir processing practice.
Friction stir processing involves rotating and translating a tool
shaft with an axially terminal shoulder and at least an axially
extending probe or pin and pressing the rotating tool into the
particle-containing surface of the workpiece. The pressure of the
rotating tool, engaging the surface, heats, softens, and
plasticizes the metal surface. The probe (or pin) is sized to
penetrate into the workpiece surface to a desired depth when the
shoulder of the friction stir tool engages the surface. The
rotating tool is also translated over the surface as necessary to
mix the particulate material into the intended surface region of
the workpiece. The rotating friction stir tool thus mechanically
deforms the particle-laden surface without melting the surface
metal. When the tool has been moved from a treated surface region,
the particles have been stirred into the processed metal of the
workpiece. The friction stir treated surface layer may be rough and
it may be machined or polished to leave a finished,
particle-modified surface region.
The depth of the surface particle receptacle cavities will
typically be no greater than the length of the probe to ensure full
participation of all the applied particulate material in the
particle-modified surface region. The length of a probe on a
friction stir tool in embodiments of this invention is usually no
greater than about three millimeters.
As stated above, the particulates in the surface layer may be of
higher hardness than the workpiece and intended to convey wear
resistance, or have lubricating characteristics and be intended to
convey lubricity to the workpiece or a combination of both in
predetermined proportion. The resulting concentration of
particulates is established by knowledge of the stored quantity of
particulates and the stirred volume and may be adjusted
substantially as required through control of either parameter.
The finished workpiece may then be employed to utilize the
properties now provided by the friction stir incorporated
particles.
Other objects and advantages of the invention will be apparent from
a description of illustrative embodiments which follows in this
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows, in perspective view, a typical sheet metal component,
a vehicle decklid.
FIG. 2 shows, in side sectional view, a die set suitable for
forming of the decklid of FIG. 1 showing the decklid in position in
the die set.
FIGS. 3 A-F show a sequence of operations for practice of the
subject invention. These figures illustrate enlarged views of
fragmental sections of die insert 10 that is illustrated in the die
set of FIG. 2. For simplified illustration the surface of the die
section in each of these views is shown as flat.
FIG. 3A shows a die insert section extending from the die surface
and extending partially into the die interior.
FIG. 3B shows the die insert section of FIG. 3A after processing to
introduce slots or scores into the surface.
FIG. 3C shows the die insert section of FIG. 3B after particulate
matter has been applied to the die surface.
FIG. 3D shows the die insert section of FIG. 3C during friction
stir processing.
FIG. 3E shows the die insert section of FIG. 3D at the conclusion
of friction stir processing.
FIG. 3F shows the die insert section of FIG. 3E after die
finishing.
DESCRIPTION OF PREFERRED EMBODIMENTS
In forming of sheet metal components through interaction with a
die, regions of relatively high forming pressure will be created at
particular locations in the die.
This may be illustrated by review of the process employed in
forming the decklid 100 shown in FIG. 1 which is formed from
decklid blank 100' shown in FIG. 2. The decklid is a closure member
for a storage compartment on an automotive vehicle. The most
significant forming challenge for this part is the bend feature 110
which transitions upper deck surface 111 to vertical deck surface
113. FIG. 2 shows an upper die 120, a lower die 130 and decklid
blank 100' in representative orientation and relative position
during forming of the decklid. To balance loads and minimize sheet
metal movement or sliding across die surface 12 during forming the
decklid blank 100' is tipped such that surfaces 111 and 113 are
disposed at generally equal angles to the vertical. Thus bend 110,
contacted by die surface 12, is positioned at the highest point of
the lower die 130 and is therefore subjected to maximum pressure
during the forming operation.
Sheet metal forming dies are largely monolithic devices frequently
fabricated of ferrous materials like cast iron. However, it is
common in regions of high load or high wear such as die surface 12
located as shown in FIG. 2 to locally replace some portion of the
cast iron with a die insert 10 of a more durable material such as
tool steel. Practice of this invention anticipates that a similar
procedure will be followed and subsequent description will focus on
the application of the invention to only insert 10 of lower die 130
without restriction to its application to a similar high load
region in a monolithic die. Further, the practice of this invention
comprehends its application to ferrous die materials generally
without restriction to specific materials such as cast irons or
tool steels. Still further it is assumed that any purely thermal
treatments such as flame hardening or furnace hardening may be
applied subsequent to the process described here.
The process, which generally entails applying friction stir
processing to disperse and distribute particles or particulate
matter in a surface layer of a body may best be understood by
reference to FIGS. 3 A-F.
FIG. 3A shows a perspective view of a small portion of surface 12
of die insert 10 illustrating the die surface 12 and a homogeneous
chemical composition 16 as it extends into the die interior.
The intent is to distribute through friction stir processing a
quantity of particulate matter within the body of die insert 10 to
a depth .delta. which is substantially equal to the length of pin
37 of the friction stir tool 22 (see FIG. 3D). Since the volume of
particulates which may be stably positioned on surface 12 is
limited, substantial dilution of the particulates will necessarily
occur as these particulates are distributed through the stirred
volume. Thus repeated sequences of surface loading of particulates
and stirring may be required to fully charge the particulates into
the surface region to the desired depth and at the desired
concentration. To better enable processing with a single stirring
operation, the surface 12 is prepared by application of any of a
number of metalworking processes to create surface features and
recesses capable of temporarily storing larger volumes of
particulates as shown in FIG. 3B.
In FIG. 3B the die is shown after a metal working process has been
used to generate these recessed surface features, here shown as
slots 14 such as might have been generated, by a saw, an end
milling cutter or slot milling cutter. This is however for purpose
of illustration only and the surface features can be generated by
any appropriate process including knurling, shot blasting,
grinding, turning or drilling. Similarly, the surface features in
FIG. 3B are depicted as having similar orientation, but this is not
a required characteristic and is shown in this manner for
convenience only. The depth of the recessed surface features should
be suitable for temporarily retaining and storing sufficient
particulate material to achieve the desired surface loading or
concentration of particles in the stirred surface. However, the
depth of the surface features should not exceed the length of the
probe, typically about 3 millimeters, on the friction stir tool to
assure contribution from all the applied particles.
The surface preparation depicted in FIG. 3B is intended to create
surface features capable of temporarily storing particles prior to
friction stir processing. In FIG. 3C, particles 20 are shown
loosely packed into slots 14. It will be appreciated that the
dimensions of particles 20 and slot 14 are related. The surface
features are intended for temporary storage of the particles and
thus the scale of the surface features should be, at a minimum,
slightly larger than the size of the particles so that at least one
particle may be fully accommodated in the feature. Since the
dimensions of the surface features will not necessarily be regular,
this requires that the minimum dimension of a surface feature be at
least no less than the particle size and preferably somewhat larger
to enable better particle packing. For example, slots with a width
of about 1 millimeter would be suitable for retaining and storing
40 mesh particles which are approximately 0.42 millimeters in
size.
With the die surface layer loaded with particles a friction stir
tool interacts with the particles and the die matrix to mix and
intersperse the particles with the die matrix material. As shown in
FIG. 3D the friction stir tool 22, rotating in a direction
indicated by arrow 25, traces a path generally indicated as 30 and
illustrated by the partial set of representative individual lateral
motions indicated by 31, 32, 33, 34 and 35 so that the entire
relevant surface is processed. The friction stir tool comprises a
generally cylindrical shank 36 with a generally cylindrical pin 37
coaxial with it and extending from shank 36 to create a shoulder 39
on the underside of shank 36. A friction stir tool with more than
one pin can also be used to effect better mixing; the pins may be
of unequal lengths. For good mixing the pin diameter should be no
smaller than the spacing between surface features 14. More
preferably, for achieving a generally-uniform particle dispersion
in the stirred volume with minimal stirring it is preferred that
the pin diameter be greater than the spacing of the surface
features. By way of example a representative pin diameter is about
6 millimeters.
Friction stir processing requires that rotating pin 37 be plunged
into the die surface 12 until shoulder 39 contacts the surface.
Then the frictional heating generated by the shoulder and the pin
will raise the temperature of the die matrix, reducing its flow
stress and increasing its plasticity. In this state the stirring
action of the pin will intermingle the die matrix and the
particulate matter 20 to create a composite 40 comprising an
intimate mixture of die matrix and particles. In general the pin
will have surface features to promote uniform stirring. These are
well known in the art and are not detailed here. Similarly it is
well known in the art that friction stir processing in ferrous
alloys can generate temperatures of about 1000.degree. C. and
above, which imposes severe thermal demands on the pin 32 requiring
that it be fabricated of, for example, cubic boron nitride for
durability.
The tool is then traversed across the surface of the insert in
suitable manner to expose the entire surface to the stirring action
of the tool and thereby achieve a generally uniform concentration
of particles in the stirred volume. If necessary, multiple
traverses may be employed and a variety of traverse paths may be
followed. Similarly the traverse paths may be oriented, relative to
any regular surface features such as those shown at 14, in whatever
aspect, parallel, orthogonal or inclined, results in the most
homogeneous concentration.
In general, heating and stirring will primarily occur in the region
between the shoulder 39 and the end of the pin 38. Thus, this
dimension should be chosen in expectation of the required depth of
the composite 40. Similarly the particle volume fraction of the
composite 40 should be chosen in expectation of its anticipated
performance.
Achieving a desired particle volume fraction may be most easily
achieved through control of the volume of particles added to the
surface and may be more readily achieved by having the volume of
the surface features (e.g. 14 in FIG. 3B) be generally equal to the
desired particle volume fraction after making appropriate allowance
for the expected packing fraction. While the scale of the roughness
may be chosen arbitrarily, primarily lateral stirring will be
required if, for example, the depth of slots 14 approximates the
desired depth of the composite layer (shown as .delta.in FIG. 3E).
For a uniform distribution of the added particulate materials the
friction stir paths should be sufficiently overlapped.
It will be appreciated from the foregoing that because of less than
complete particle packing i.e. a particle packing fraction of less
than 100%, the stirred volume of matrix and particles will be less
than the initial volume of the die matrix itself. Hence it is
desirable that the initial die surface 12 lie slightly above the
desired or finished die surface (indicated by 112 in FIG. 3F) to
minimize die finishing requirements.
The choice of particles will be dictated by the performance
objective and may include hard, rigid particles like silicon
carbide, aluminum oxide or cubic boron nitride or lubricious
particles like hexagonal boron nitride, or if oxidation can be
controlled during the stirring process, for example by conducting
stirring under an inert atmosphere, molybdenum disulphide or
graphite. Alternatively an appropriate combination may be drawn
from each class.
FIG. 3F shows the final result after the tool has traversed the
entire workpiece surface--a composite layer 40 of depth .delta.,
where .delta. is approximately the distance between shoulder 39 and
the end 38 of pin 37, that is where .delta. is approximately the
pin length.
In general, the surface of the composite layer after friction stir
processing will not be a suitable die surface and some additional
surface finishing, typically grinding and polishing will be
required to render the finished die surface 112 illustrated in FIG.
3F.
The invention has been particularly described in terms of its
utility for sheet forming dies. However it will be appreciated by
those skilled in the art that there are numerous applications, such
as bearings or slideways, for which it might also be advantageous
to modify the surface frictional characteristics. Further, although
the invention described has been directed to metallic substrates,
thermoplastic polymers, which are generally soft and exhibit poor
wear resistance may also be plasticized using friction stir
processing and thus might beneficially be processed as described
above.
In practice of this invention, grooves of approximate dimensions
0.5 millimeters wide and 1-2 millimeter deep would be prepared on
some selected portion(s) comprising up to 10% of the sheet-metal
contacting surface of a die or die insert fabricated, for example,
of P20 tool steel. At least the regions incorporating the grooves
would protrude beyond the intended die surface to provide
opportunity for shaping and suitably finishing the surface modified
region. The grooves are filled with a pre-selected quantity and
blend of friction-modifying particles of size and shape suitable
for accommodation in the grooves.
The friction-modifying particles are stirred into and incorporated
in the die surface by passage of a friction stir tool participating
in a friction stir process, the tool pin typically extending only
to a depth generally comparable to the depth of the grooves. A
suitable tool material for lubricious particles and low melting
point die matrix materials is AISI H13, but more abrasive,
wear-resistant particles like silicon carbide would necessitate use
of ceramic or metal-matrix composite tools.
It is preferred that the stirring tool traverse the region more
than once. These traverses can be done in the same direction as,
opposite or at an angle to the direction of the previous pass. A
typical stirring depth will be up to 3 millimeters informed by the
depth of the grooves as previously discussed.
Friction stir processing will result in displacement of material
from the surface and creation of a cavity at the point of tool exit
unless an adjustable pin tool is used. In the case of die, the exit
location may be positioned in a region of the die which either will
not contact the sheet metal or, less preferably, a region of the
die which experiences only a low forming pressure. The tool exit
hole may be filled by friction welding the hole with a suitable
plug or by arc welding. For an insert, it is preferred to make the
insert sufficiently oversized to accommodate the entry and exit
locations on the excess portion of the insert and remove the excess
by machining prior to incorporating the insert in the die.
Finally the friction stir processed surface of the die or insert
would be finished by machining and/or grinding and/or polishing to
achieve the designated final dimensions and surface finish. The
extent of such finishing being at least sufficient to remove any
irregularities resulting from friction stir processing and render a
continuous surface contour suitable for sheet metal forming.
Thus while the best modes for carrying out the invention for a
specific application have been described in detail, those familiar
with the art to which this invention relates will recognize various
alternative opportunities for practicing the invention within the
scope of the appended claims.
* * * * *